AN AGONISED appeal for help from a Thai woman who is being held against her will in a brothel by gangsters somewhere in Japan appeared in a Japanese newspaper this week. But no one, apart from her captors, knows where the woman is being held.

'I am a prisoner of the yakuza (gangsters), and forced into prostitution. This is hell on earth,' she wrote in a letter to her father. 'If you lose contact with me, you will know I have been killed.'

The letter, written in Thai, was apparently smuggled out of the brothel by one of the woman's clients, and had a postmark from Osaka, Japan's second largest city. A facsimile of the letter appeared in the Yomiuri newspaper, with the woman's name deleted - police fear her captors might kill her if they realise police are looking for her.

The case highlights the growing sex trade between South-east Asia and Japan, which is controlled by underworld syndicates able to circumvent the Japanese police and the immigration service. Sources in Bangkok estimate some 70,000 Thai women are working as prostitutes in Japan, many of them lured into the trade against their will with offers of jobs as maids or factory workers. Once in Japan their passports are confiscated, and speaking no Japanese and often no English, they are at the mercy of gangsters who run the sex trade.

The woman, 31, comes from Nan province in Thailand. The last time her family heard from her was August 1992, when she was preparing to go to Singapore, where a job placement agency had supposedly found her a job as a maid. She said she would send money to her relatives, who are subsistence farmers.

Instead she was taken to Japan, where she was brought in on a false visa and then 'sold' to gangsters, who took her passport. After a drive of 'about two hours' in a van with no windows she was pushed into a bar where she has been confined ever since.

According to her letter, she is not allowed out, cannot get to a telephone, and fears that if she tries to escape, 'I will be murdered'. The letter was posted in March and reached her father in April. He gave it to a Japanese businessman in Thailand, along with a photograph of his daughter. This was given to the Japanese police, who are trying to find the woman.

The letter gives the name of the bar, and says it is on an island. There are no other details. The letter ends: 'I want to go back to Thailand and live in freedom.'